Compared to classrooms in some countries, United States' classrooms tend to be informal.
There are, however, some very important basic rules:
Before class:

Do your homework!
Read critically; form your own opinions
Review your notes
from the previous lecture and reading for the day
Communicate immediately with professors
about any study problems
Focus on the task at hand before class:
take a moment of silence to gather your thoughts and mentally prepare yourself to the topic
Write any objectives
that come to mind at the head of your notepaper:
preparing for an up-coming test,
understanding a particular concept,
gaining a good foundation on a topic
understanding or reviewing the readings
In Class:

Arrive on time for class.
Professors do not take lateness lightly
Position yourself in the classroom
to focus on the subject matter; consider the best location for:
listening
asking questions
seeing visual materials
discussing--not only with the teacher but also your classmates
Avoid distractions
that may interfere with your concentration
(daydreaming, looking around the room, talking to a friend, passing notes, dozing)
Evaluate as you listen:
Decide what is important and should be placed in your notes and what can be left out;
Listen long enough to be sure you understand what was said before writing.
Ask clarifying questions (but wait for "breaks" in the instructor's stream).
Review your class objective(s) throughout the class period
Did your objective(s) mesh with the instructor's introductory remarks?
Has the class digressed from stated objectives, yours or the instructor's?
Make a to do list including
assignments;
reviewing difficult concepts;
joining study groups;
making appointments with a study pal, tutor, or the instructor.
One resource often overlooked is a classmate who seems to have a good grasp of the material. If it seem appropriate, seek the individual out for help.
Periodically ask yourself if the course is meeting your objectives. If you find yourself dissatisfied with a particular class or the course in general, make an appointment with the instructor to discuss your expectations. The earlier the better.